sha1-name.c: remove implicit dependency on the_index

sha1-name.c: remove implicit dependency on the_index

This kills the_index dependency in get_oid_with_context() but for
get_oid() and friends, they still assume the_repository (which also
means the_index).

Unfortunately the widespread use of get_oid() will make it hard to
make the conversion now. We probably will add repo_get_oid() at some
point and limit the use of get_oid() in builtin/ instead of forcing
all get_oid() call sites to carry struct repository.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>